There has heretofore been known a drive apparatus of a vehicle including a differential device through which an output of a drive power source, such as an engine or the like, is distributed to a first electric motor and an output member, and a second electric motor disposed between the output member and drive wheels. Such drive apparatus for a hybrid vehicle is disclosed for example in a Patent Document 1 and a Patent Document 2. It includes a differential mechanism being comprised of a planetary gear unit and performing a differential action for mechanically transmitting a major part of power outputted from the engine to drive wheels. A remaining part of power from the engine is electrically transmitted from the first electric motor to the second electric motor using an electrical path.
Thus, the drive apparatus operates a transmission of which shifting ratio is electrically changed, for example as an electrically controlled continuously variable automatic transmission. The drive apparatus is controlled by the control device so that the vehicle runs with the optimum operation state of the engine, thus improving fuel consumption, i.e., mileage.
Patent Document 1: JP 2001-339805A
Patent Document 2: JP 2003-301731A
Meanwhile, the hybrid vehicle driving apparatus disclosed in the aforementioned Patent Document 1 generates reaction torque in the first electric motor in accordance with the engine torque that is necessary to serve as an electrically operated continuously variable transmission. This reaction torque that is generated against the engine torque by the first electric motor is limited by the rated output and the amount of heat (temperature) generated by the first electric motor. Accordingly, from the viewpoint of preventing an increase in the size of the first electric motor, it may be necessary to limit the engine torque in order not to exceed the reaction torque that can be generated by the first electric motor. Alternatively, the original output of the engine that is installed in the vehicle may be limited.
In addition to this, generally, continuously variable transmissions are known as apparatuses that improve the fuel efficiency of vehicles, while, step variable transmissions such as gear shifting transmissions are known as apparatuses that provide high transmitting efficiency. Unfortunately, a power transmission mechanism that has both these advantages has not yet been developed. For example, the hybrid vehicle driving apparatus as disclosed in the aforementioned Patent Document 1 or 2 includes an electrical path for the electrical energy to transmit from the first electric motor to the second electric motor, i.e., a transmission path that transmits a part of the drive force in the form of electrical energy. For this reason, the first electric motor is likely to increase in size in accordance with the increase of engine output, and the second electric motor that is driven by the electrical energy provided from the first electric motor is also likely to increase in size. As a result, there is a problem in that the size of the driving apparatus increases.
Also, since a part of the engine output is converted into electrical energy and then transmitted to the drive wheels, fuel efficiency may actually decrease under some vehicle operating conditions, such as operating at high speed. Furthermore, in the case where the aforementioned power distributing mechanism is used as a transmission that electrically changes a shifting ratio, e.g., a continuously variable transmission called an electric CVT, a similar problem exists.
In the conventional vehicle driving apparatus disclosed in the aforementioned Patent Document 1, the differential mechanism or the first electric motor has a torque capacity limit that is derived from its configuration. Accordingly, the differential mechanism or the first electric motor is likely to increase in size in accordance with the increase of engine output, which in turn causes a disadvantage in a vehicle. For example, in a type of conventional vehicle driving apparatus that distributes engine output to the first electric motor and a transmitting member, since the first electric motor controls a transmission that electrically changes a shifting ratio, the first electric motor bears the reaction torque against the engine output torque (hereinafter, referred to as “engine torque”). Accordingly, the reaction torque capacity borne by the first electric motor is required at a level that is in accordance with the engine torque provided. For example, the reaction torque capacity borne by the first electric motor increases in accordance with the increase of the engine torque required to provide a desired acceleration performance, and the like. Consequently, as the output of an engine increases, the first electric motor also increases in size.
Hence, it is conceivable that, in order to protect the differential mechanism or the first electric motor without increasing the size thereof, the engine torque must be temporarily limited to fall within the maximum engine torque range that can be supported by the maximum reaction torque capacity that the first electric motor can bear. However, in this case, this limitation reduces the torque that is transmitted to the drive wheels, which in turn affects acceleration performance. As a result, the desired acceleration performance required may not be provided, particularly while starting the vehicle or undertaking passing maneuvers.
The present invention has been developed in consideration of the above situations, and has an object to provide a control device that is used for a vehicle driving apparatus that includes a differential mechanism performing a differential action, and is capable of reducing the size of the driving apparatus, or of improving the fuel efficiency and increasing the output torque.
A second object of the present invention is to provide a control device that is used for a vehicle driving apparatus that includes a differential mechanism for distributing engine output to a first electric motor and a transmitting member, and an electric motor that is operatively connected to the wheels, and is capable of ensuring the acceleration performance of a vehicle.